A porous resin membrane having microscopic through holes can be produced by stretching a membrane made of fluororesin consisting mainly of polytetrafluoroethylene (hereinafter referred to as PTFE). Formed mainly of PTFE, the porous resin membrane has excellent chemical and heat resistance, so that it is used as a filtration membrane (a filter) for filtering microscopic particles.
As stated in, for example, Patent Literature 1, a porous resin membrane made of PTFE can be produced by the following process. First, an extrusion aid is mixed into a PTFE fine powder. Then, the mixture is paste-extruded to obtain a desired shape. The formed product is heated to attain a semibaked state (a state in which unbaked portions and baked portions are mixed together, the state being produced by melting only part of the formed product). Finally, the processed formed product is stretched to become porous. In the above description, the PTFE fine powder is produced, first, by polymerizing tetrafluoroethylene by using emulsion polymerization or otherwise to form PTFE particles (primary particles) having a diameter of 0.15 to 0.35 μm and, then, by granulating them to achieve a diameter of several hundred to several thousand micrometers. In the process stated in Patent Literature 1, however, PTFE is stretched in a semibaked state, in which the PTFE is not completely melted. Consequently, the process is, basically, powder molding and therefore gaps are present between particles before the stretching. As a result, it is difficult to produce a microporous membrane having a uniform pore diameter.
Patent Literature 2 has disclosed that a thin nonporous fluororesin membrane having few defects such as voids and cracks can be obtained through the following process. First, a fluororesin dispersion produced by dispersing a fluororesin powder such as a PTFE fine powder in a dispersion medium is applied onto a smooth film. Then, the dispersion medium is evaporated, and the fluororesin powder is baked to melt it completely. The literature states that subsequent to the above process, by stretching the thin nonporous fluororesin membrane to render it porous, a thin fluororesin membrane having micropores with a uniform pore diameter, having high porosity, and being free from defects can be obtained and that the thin fluororesin membrane can be favorably used as a filtration membrane for microscopic particles.
In addition, Patent Literature 3 has stated that by bonding a thin fluororesin membrane formed as described above with a stretchable supporter and subsequently by stretching them at lower than 30° C., a thin porous fluororesin membrane can be produced that has microscopic pores capable of removing microscopic particles even the membrane is a thin membrane having a thickness of 50 μm or less and that has high porosity. This process can be applied to the production of, particularly, a thin porous fluororesin membrane having a mean flow pore diameter of 45 nm or less.